Flux gate magnetometer including backing plate

ABSTRACT

A magnetometer having a flux sensing head including a core and a magnetically permeable backing plate opposite the core with second harmonic driven detection coils on the core for sensing a static magnetic field extending between the core and the backing plate. The magnetometer is designed for detecting the presence of magnetic inks between the detection head and the backing plate.

D United States Patent 51 3,643,155 Riddle et al. 1 Feb. 115, W72

[54] FLUX GATE MAGNETOMETER [58] Field of Search ..324/34,43; l79/l00.2 C, 100.2 CF

INCLUDING BACKING PLATE [56] References Cited [72] Inventors: John B. Riddle; Arndt Bcrgh, both of Palo Alto; Charles 0. Forge, Cupertino, all of UNITED STATES PATENTS 2,999,135 9/196] Wiegand .324/43 ux [73] Assignee: Micro Magnetic Industries, Inc., Palo Alto,

Calif. Prlmary bxammer-Alfrcd E. Smnh Att0rneyLimbach, Limbach & Sutton [22] Filed: Apr. 26, 1968 [21] Appl. No.: 758,167 [57] ABSTRALT A magnetometer having a flux sensing head including a core Related [15- Applica ion D and a magnetically permeable backing plate opposite the core with second harmonic driven detection coils on the core for [62] Dwlslon of f 1 Jul); 1966 sensing a static magnetic field extending between the core and g a dmslon 0 the backing plate. The magnetometer is designed for detecting l 3 the presence of magnetic inks between the detection head and the backing plate. [52] US. Cl ....324/43, 179/1002 {51 Int. Cl ..G0lr 33/04 6 Claims, 11 Drawing Figures PATENTEDFEB 15 m2 3. 643 l 55 saw 1. or 7 3 53 ARNDT a, BERGH CHARLES 0. FORGE A 7' TOPNE VS PATENTEUFE! \5 m2 $630155 SHEET 2 OF 7 INVENTORS JOHN B. RIDDLE ARNDT B4 BERG CHARLES O FOR ATTOPNEVS PATENTEDFEB 15 I972 3, M3, 1 55 sum 3 or 7 Ill n9 INVENTORS JOHN B. RIDDLE ARNDT a BERGH CHARLES ov FORGE WILM A T TO/P/VEVS PAIENTEUFEB 15' 1972 3,643,155

SHEET '4 OF 7 INVENTORS JOHN RIDDLE ARNDT' B, BERGH CHARLES O FORGE ATTORNEYS PATENTEDFEB 15 I972 3,643,155

SHEET 5 OF 7 INVENTORS JOHN. B. RIDDLE .ARNDT BEERGH CHARLES o. FORGE ATTORNEYS PMENIEUF EB 1 5 15?? SHEET 7 OF T! INVFJVTORS JOHN B. RIDDLE ARNDT B. BERGH CHARLES O FORGE A T TOP/V575 FLUX GATE MAGNETOMETER INCLUDING BACKING PLATE This application is a division of our copending application Ser. No. 596,024, filed July 25, 1966, now US. Pat. No. 3,39l,388, which in turn was a division of our copending application Ser. No. 149,799, filed Aug. 23, 1961, now US. Pat. No. 3,280,974.

It is another object of the invention to provide apparatus for identifying pieces of printed matter in which a detection element is moved with respect to the pattern of indicia on a piece of printed matter and generates an electrical detection signal which is a parameter of the pattern but where the detection signal bears the same parametric relation to the pattern of in dicia regardless of the relative speed with which the detection element and pattern are moved with respect to each other.

It is another object of the invention to provide apparatus for recognizing the validity and denomination of printed currency by magnetizing the ink on said currency and then analyzing the pattern of magnetic flux adjacent to the printed indicia on the currency.

It is another object of the invention to provide such currency recognition apparatus in which means are provided for magnetizing the ink on bills in a direction perpendicular to the face of the bill and then moving the face of the bill past a magnetic flux measuring device, or magnetometer, to produce a detection signal from the magnetometer which is a parameter of the flux sensed by the magnetometer while the bill moves with respect thereto.

As used herein the term magnetometer means a device for statically sensing magnetic flux in space and generating an electrical signal which is a parameter of the flux sensed. The magnetometer contains a head portion for sensing static flux in a local area of space and a power means portion connected to the magnetometer for generating an electrical signal which is a parameter of the flux sensed by the head portion. The magnetometer is thus to be distinguished from what may be called a dynamic head which is a magnetic measuring device which generates an electrical signal as a parameter of the rate of movement of static magnetic flux adjacent to the device, the movement of flux resulting from the movement of the dynamic head in a static magnetic field, the movement ofa magnetic field past a stationary dynamic head, etc. The use of the magnetometer instead of a dynamic head greatly reduces control and synchronization problems in the device, since with a magnetometer, an unknown bill may be examined and correlated to a known counterpart in space without reference to time whereas the use of a dynamic head may require accurate correlation with space and time.

It is another object of the invention to provide such apparatus in which, at each instant while the magnetized bill moves with respect to the magnetometer, the magnetometer measures magnetic flux from only a very small fragment of the pattern of indicia on the bill so that the magnetometer sees the pattern of indicia with maximum detail.

It is another object of the invention to provide an improved flux sensing head portion for the magnetometer used in apparatus of this type whereby the head portion will be able to see greater detail in a pattern of printed indicia than has been possible heretofore.

It is another object of the invention to provide apparatus for recognizing the validity and denomination of printed currency in which means are provided for magnetizing the ink on currency in a direction perpendicular to the face of the currency and then move the currency with respect to a magnetometer which will generate a detection signal which is a parameter of the pattern of indicia.

It is another object of the invention to provide such apparatus in which the magnetometer has extreme sensitivity both in its ability to detect minute quantities of magnetic flux and in its ability to detect such quantities of flux emanating from minute physical areas.

It is another object of the invention to provide such apparatus having means for adjusting the magnetometer output to a starting condition automatically prior to each time it is used for recognizing a piece ofcurrency.

In the attached drawings:

FIGS. 1 to 6 are drawings of physical support and transport means employed in the invention;

FIG. 1 being a side elevation of such transport means,

FIG. 2 being a fragmentary view taken along the plane and in the direction indicated at 22 in FIG. 1 and illustrating specifically the drive reversing means shown in FIG. 1,

FIG. 3 being a side elevational view of the apparatus taken from the opposite side as FIG. 1,

FIG. 4, being a vertical section of the apparatus taken along the plane 4-4 of FIGS. I and 3,

FIG. 5 being a view in end elevation of the apparatus, and

FIG. 6 being a longitudinal section of the apparatus taken along the plane 66 of FIG. 5;

FIGS. 7 to 9 are drawings of three alternative forms of improved magnetic flux measuring heads forming a part of the invention and forming an important new subcombination having substantial utility and unique features apart from its use with other components of the invention;

FIG. 7 being a perspective view of a preferred form of head,

FIG. 8 being an end elevation of an alternative form of irn' proved head, and

FIG. 9 being an end elevation of still another alternative form of improved head;

FIG. 10 is a composite drawing and circuit diagram of a preferred system of the invention showing the apparatus of FIGS. 1 to 9 interconnected with electronic means for rendering the apparatus operative automatically and efficiently;

FIG. 11 shows, first in the upper portion, parts of four waveforms which might be obtained by recording the detection signal produced by the magnetometer shown in FIG. 10 while valid bills of four different denominations were examined by the apparatus and showing, second in the lower portion, four different shapes which would be used for the switching conductors on the rotatable drum shown best in FIGS. 4, 5 and 10, when the four different denominations indicated in the upper part of the drawing were to be recog nized. For simplicity of illustration and description, the curves and shapes are aligned in synchronization vertically of the drawing, and the shapes in the lower part: of the drawing are i! lustrated schematically;

MECHANICAL SUPPORT AND TRANSPORT APPARATUS Referring now in detail to the drawings and particularly to FIGS. 16, the apparatus illustrated therein for transporting pieces of printed currency to be examined comprises a frame 20 on which are mounted four axles 21, 22, 23, and 24, each of the axles carrying a roller 25 with belts 26 and 27 extending over horizontally disposed pairs of rollers 25 as indicated. The belts 26 and 27 are preferably made of thin sheets of suitable plastics which contain no magnetizable substances.

The axle 23 (as best seen in FIG. 3) extends laterally from the frame 20 and may be attached to a suitable motor to be driven thereby, the axles 22 and 23 carrying gears 28 and 29 thereon by which the two belts 26 and 27 are driven in synchronization. A loading tray 30 (see FIG. I) is mounted adjacent to the bite of the rollers 25 on axles 21 and 24 to deliver to the belts 26 and 27 pieces of printed currency inserted in the tray 30. A start switch 31 actuated by a photoelectric cell 32 is mounted adjacent to the tray 30 and positioned to be closed when a piece of currency is inserted in the tray 30 interrupting light transmission to the cell 32', the switch 31 is connected to the motor (not shown) which drives the axle 23 to start operation of the motor. For reasons that will appear hereinafter the width of the tray 30, measured in a plane perpendicular to FIG. I, is preferably substantially equal to the width of pieces of printed currency which are to be examined by the apparatus whereby pieces of printed currency are fed to the belts 26 and 27 along a path generally parallel to the lengths of the pieces of currency.

A pair of horseshoe magnets 33 and 34 are mounted adjacent to opposite edges of the belts 26 and 27 rigidly attached to the frame 20, and a pair of magnetic pole pieces 35 and 36 are rigidly attached to like ends of the magnets 33 and 34 whereby one pole piece becomes a north pole and the other pole piece becomes a south pole with a magnetic field extending between the pole pieces 35 and 36 generally perpendicular to the belts 26 and 27. It should be noted that the belts 26 and 27 convey pieces of printed matter between the pole pieces 35 and 36 so that magnetizable particles in the ink with which the currency is printed are magnetized in a direction perpendicular to the faces of the piece of currency, one face of the piece of currency becoming a north pole and the other face becoming a south pole.

A pair of brackets 37 and 38 are rigidly attached to the frame 20 by dowels 39 and screws 40 with the brackets 37 and 38 being positioned on opposite sides of the adjacent courses of the belts 26 and 27 so that pieces of printed matter conveyed by the belts will pass between the brackets 37 and 38 subsequent to their passage between the magnetic pole pieces 35 and 36. Accordingly it should be noted that normal operation of the motor which drives axle 23 causes rotation of the rollers 25 and the gears 28 and 29 in the direction indicated by the arrows 41. As explained in greater detail hereafter, a mag netic flux measuring head portion of a magnetometer is mounted on the brackets 37 and 38 to measure the pattern of magnetic flux surrounding printed indicia on currency supported by the belts 26 and 27. The brackets 37 and 38 are mounted centrally of the width of belts 26 and 27 so that the head portion of the magnetometer will see the same strip of a bill regardless of which face of the bill is placed upwardly in the tray 30.

It will thus be noted that this apparatus provides means for maintaining the pole pieces 35 and 36 and the flux sensing head in brackets 37 and 38 in stationary positions fixed with respect to each other while a piece of currency to be examined is moved with respect to them. While it may be possible to reverse this procedure and move the pole pieces and flux sensing head while holding the piece of currency stationary, it has been found that the manner of operation illustrated is much more desirable since the reverse procedure necessitates excessive shielding of the flux sensing head, since when the head is moved it is moved through the earths magnetic field, and it has been found that the quantities of magnetic flux sensed by the head from pieces of currency are substantially less than the quantities of flux in the earth magnetic field through which the head would have to pass if it were moved.

As explained in connection with FIG. hereafter, passage of a piece of currency along the path between the pole pieces 35 and 36 and the brackets 37 and 38 produces a detection signal which is a parameter of the pattern of printed indicia on the currency. This detection signal is compared continuously with a physical counterpart derived from a pattern of printed indicia on a valid piece of currency of known denomination, favorable comparison effecting recognition of the test bill, and unfavorable comparison effecting rejection of the test bill. When the test bill is to be rejected, the direction of movement of the motor driving axle 23 is reversed to thereby deliver the test bill back to the end of the apparatus adjacent to the tray 30.

Synchronization of comparison between the detection signal and the physical counterpart of a known bill is effected by mounting the physical counterpart on a rotatable drum 42 (see FIGS. 2-6), rotating the drum 42 by means of the mechanical linkage shown in FIG. 2, and electronic synchronization means to be described in greater detail hereinafter. The drum 42 is rotatably supported on the frame by means ofa pair of stub shafts 43 and 44. The drum 42 is provided with an internal, generally conical, friction lining 45 engageable with a clutch surface 46 on a disc 47, with the surfaces 45 and 46 being resiliently urged together by means of a compression spring 48 engaging an internal shoulder 49 in the drum 42. The drum 42 is freely rotatable on the shafts 43 and 44 and is caused to rotate solely by frictional contact between the surfaces 45 and 46. The disc 47 is rigidly attached to the shaft 44, and the shaft 44 carries a drive gear 50 rigidly attached thereto.

A shaft 51 is mounted on the frame 20 and carries an inner gear 52 (see FIG. 2) and an outer gear 53 with the gear 53 being engaged with the drive gear 50. A reversing shoe 54 is frictionally mounted on the shaft 23 by means of bolts 55 so that normal rotation of the shaft 23 urges counterclockwise rotation of the shoe 54, and reverse movement of the shaft 23 urges clockwise rotation of the shoe 54. The shaft 23 carries a gear 56 thereon, and the shoe 54 carries a forward drive gear 58 and a reverse drive gear 59 thereon with both of the gears 58 and 59 in permanent engagement with the gear 56. The gears 58 and 59 are arranged as indicated so that l) the gear 58 engages with gear 52 when the shoe 54 rotates counterclockwise to thereby effect rotation of drive gear 50 in the direction of arrow 60, and (2) the gear 58 disengages from gear 52 and the gear 59 engages with gear 50 responsive to clockwise rotation of the shoe 54 to thereby rotate the drive gear 50 in the direction of arrow 60 regardless of the direction of rotation of the shaft 23. This constant directional rotation of the drive gear 50 and hence the drum 42 regardless of the direction of rotation of the shaft 23 permits the apparatus to recognize a recognizable piece of printed currency regardless of the manner in which the piece of currency is inserted into the tray 30. As indicated above, the magnetic detection head in the brackets 37 and 38 is mounted to scan a longitudinal track along the center of a piece of currency between the belts 26 and 27 so that the head will see the same pattern of indicia regardless of which face of the bill is placed upwardly in the tray 30. Also, the bill can be reversed end for end in the tray 30 without preventing the apparatus from recognizing it since if the head sees the pattern of indicia backwards with reference to the counterpart on the drum 42 when a bill is moving into the machine (toward the left as viewed in FIG. 1), reversal of the shaft 23 to reject the bill will permit the head to see the pattern of indicia properly as the bill moves out of the machine (to the right as viewed in FIG. 1) suitable switching (not shown) being provided to permit the head to look at and recognize the bill while it is passing in both directions.

While synchronous drive is provided between the shaft 23 and the counterpart drum 42 through the gears of FIG. 2 and the clutch surfaces 45 and 46, this driven connection is electronically synchronized at the beginning of each test and is electronically resynchronized a plurality of predetermined times while a piece of currency is passing the magnetic flux detecting head. This resynchronization permits the apparatus to recognize all bills of any given denomination even where different spacings are found between different local areas of printed indicia on the bills ofa given denomination. This electronic synchronization is provided by the use of a plurality of mechanical stops for interrupting rotation of the rotatable drum 42 just prior to the beginning of each test and at a plurality of rotational positions of the drum during the passage of a piece of currency under the head. The mechanical stops are then released electronically responsive to the detection of magnetic flux by the head so that rotation of the counterpart drum 42 is initially synchronized and is later resynchronized responsive to the actual arrival of areas of printed matter at the magnetic detection head.

As may be seen most clearly in FIGS. 4 and 6 and in a corresponding portion of the apparatus of FIG. 14, a plurality of stop members 61 are mounted on one end of the drum 42 by means of bolts 62, and the members 61 have stop ends 63 axially spaced of the drum from each other and cooperating with a corresponding plurality of adjustable stop screws 64 so that rotation of the drum 42 is interrupted when the stop end 63 of one of the stops 61 contacts its corresponding stop screw 64. The stop screws 64 are mounted on the movable arm 65 of a stop release relay 66 so that energization of the relay 66 pulls the stop screw 64 to the left as viewed in FIG. 6 to thereby release them from the stops 61. It should here be noted that the stop screws 64 are mounted on a spring arm 67 against the resiliency of which the stop screws 64 are moved when they contact the steps 61. This movement of the stop screw 64 in opposition to spring 67 causes the opening of a pair of normally closed switch contacts 68 and the closing of a pair of normally open contacts 69. The functions of these switch contacts 68 and 69 will become more apparent in connection with the description of FIG. 10.

In addition to the switches 68 and 69 which are actuated by the drum 42, a cam-actuated stack of switches 70 (see FIGS. 1 and is actuated by a roller 700 which moves along a track on the opposite edge of the drum 42 from the stops 61. The stack of switches 70 includes seven contacts 71-77 and a pair of insulators 78 and 79. The contacts 71-73 define a singlepole double-throw switch in which the contacts 71 and 72 are closed at the starting rotational position of the drum (when the roller 70a descends into a notch 80 in the drum 42) (see FIG. 10). At all other rotational positions of the drum 42, the switch contacts 71 and 72 are open, and the switch contacts 72 and 73 are closed. A plurality of small areas 81 of the periphery of the drum 42 are provided with slight rises therein which effect closing of the switch contacts 74 and 75, and a large cam 82 is provided on the drum positioned to contact the roller 70a just before the roller 70a moves into the notch 80. When the roller 70a moves over the large cam 82, the switch terminals 76 and 77 are connected. As will appear from the description of FIG. 10, the switches 68 and 77 are employed to control the automatic logic which forms a portion of the apparatus.

FLUX SENSING HEAD PORTION OF MAGNETOMETER As mentioned heretofore an important feature of the invention is the provision of an improved magnetic flux sensing head which has sufficient sensitivity from the standpoint of measuring small quantities of flux and resolving closely spaced lines of printed matter that the apparatus of this invention is capable of recognizing pieces of currency and distinguishing between different denominations thereof by scanning the magnetic pattern of a magnetized bill. This sensitivity is obtained through the new design of a flux measuring head used in a flux gate magnetometer, that is, the type of magnetometer in which measurement of magnetic flux is accomplished by periodically varying the reluctance of a magnetic conductor with a driving magnetic field.

Referring more specifically to FIG. 7, the flux sensing head illustrated therein comprises a pair of blades 83 and 84, a magnetic shunt 85, and two pairs of magnetic conductors 86 and 87 connecting the shunt 85 to the blades 83 and 84 respectively. The elements 83-87 are preferably constructed of a very low magnetic reluctance material such as Mu-metal, and these elements define a magnetic conductor having opposite ends 88 and 89 which are beveled with the blades 83 and 84 slanting away from each other as they diverge from the edges 88 and 89. The flux sensing head shown in FIG. 7 is illustrated at several times its actual size, the edges 88 and 89 being very closely spaced together (on the order of 10-30 thousandths of an inch apart), and this magnetic conductor is enclosed within a magnetic shield 90 having a shaped aperture 91 therein with the edges 88 and 89 of the magnetic conductor positioned in the aperture 91. A backing plate 92 constructed ofa relatively low reluctance material is mounted adjacent to the edges 88 and 89 of the magnetic conductor and the aperture 91 of the shield 90 with the spacing between the backing plate and the shield and conductor being very small. In the operation of this head, magnetic flux flowing generally perpendicular to the backing plate 92 is measured as it flows through the magnetic conductor, the flow of magnetic flux through the conductor being caused by differences in magnetic potential adjacent to the two edges 88 and 89. Thus, assuming a higher potential adjacent the edge 88, flux would flow from backing plate 92 to the edge 88 upwardly through the strips 86, across the shunt 85, down the strips 87, through the edge 89 and back to the backing plate 92. Thus, it will be noted that measurable flux passes through the conductor only when a gradient of magnetic potential exists between the edges 88 and 89, and with these edges so close a very high degree of resolution is obtained with the head. To some extent magnetic flux is also caused in the magnetic conductor for instance by the flow of flux from the edges of the aperture 911 in the shield 90 to the edges 88 and 89 of the magnetic conductor, but such induced magnetic flow is very small when compared to the flow caused by differences in magnetic potential immediately adjacent to the edges 88 and 89. It should be noted that the positioning of the edges 88 and 89 within the aperture 91 of the shield 90. together with the provision of the backing plate 92, provides an extremely sensitive flux measuring device since the mag netic conductor is effected by very little flux other than the flux immediately adjacent to its edges.

The geometrical area from which magnetic flux flows to the conductor can be further controlled, when desired by shaping the backing plate 92; the provision of a ridge on the backing plate 92 immediately under the aperture 91 and parallel to the aperture 91, and the provision of such ridges extending perpendicular to the aperture 91 may be desirable in some instances,

When magnetic flux flows along the abovementioned path through the conductor, it is measured by electrical coils 93 and 94 which are wound on the strips 86 and 87 respectively. The coils 93 and 94 are connected together as indicated so that alternating current input at terminals 95 will alternately establish closed magnetic loops in the conductor first in the direction of the arrows thereon and then in the opposite direction. The establishment of these closed loops of magnetic flux especially when the input current is sufficiently high to saturate the strips 86 and 87, effectively provides switches which turn on and off the flow of measurable flux through the conductor from the backing plate 92 as described above. When the input alternating current at terminals 95 has a predetermined frequency, the path of measurable flux is turned on and off at an even multiple of the predetermined frequency, and as this measurable flux :is turned on and off, it induces current in the coils 93 and 94. When detection leads 96 are connected to the coils as illustrated, this even harmonic induced current can be detected from these terminals. Thus, with this arrangement an input current at frequency F is applied to terminals 95 and an output signal is detected at ter minals 96 at a frequency of ZF. The construction of the head as illustrated provides maximum sensitivity of the head first from the standpoint of producing large signals at terminals 96 compared to noise and secondly from the standpoint of the production of these large signals from very small magnetic sources in very small physical areas between the edges 88 and 89 of the conductor and the backing plate.

A similar magnetic head is shown in FIG. 8 where the flux gate coils 93 are provided in only one leg (83) of the magnetic conductor. A further alternative form of head is shown in FIG. 9 where the magnetic conductor is provided with only one leg and return path for magnetic flux flowing to the backing plate is providing through the shield surrounding the magnetic conductor. In this form of the head, the path of measurable flux is indicated by the arrows 97, and the path of the closed loop flux which turns the measurable :flux on and off is in dicated by the arrows 98.

For some particular applications of the magnetometer heads of this invention particularly where they are used in currency recognizing apparatus, it may be desirable to employ devices sensitive to the Mall effect as the magnetic flux measuring element in the head. In such an application where the head of FIG. 8, for instance, is to be used, the conductor strips 86 and the coils 93 may be replaced by a wafer of a material sensitive to the Mall effect with the plane of the wafer perpendicular to the path of magnetic flux in the conductor 83. An AC drive current of predetermined frequency is then applied through the wafer along the plane of the wafer, and a voltage proportional to the magnetic flux in the conductor 83 is detected across the wafer in a direction perpendicular to the direction of the drive current and having the same AC frequency as the drive current.

SYNTHETIC COUNTERPART OF KNOWN PIECE OF CURRENCY As mentioned heretofore the apparatus of FIGS. 1-10 employs a physical counterpart which is a parameter of the detection signal which would be generated by the magnetometer when a piece of printed currency of known validity and denomination was delivered to the belts 26 and 27. In FIGS. 4 and 5, the drum 42 is illustrated as carrying such physical counterparts for three different denominations of printed currency. Thus, electrical switch wipers 107 are provided for contacting pairs of switch projections 108 and 109 with the pairs of projections carried respectively by electrical conductor strips 110 and 111 for the projections 108 and conductor strips 111 and 112 for the projections 109. These conductor strips and projections form a counterpart of a 1 dollar bill. Only these conductor strips and projections are illustrated on the drum 42 in FIG. 10 for the sake of simplicity of illustration, however, additional switch wipers are illustrated in FIG. 5 for the other denominations, a pair of wipers 113 and associated strips being provided for recognition of 5-dollar bills, and a pair of wipers 114 and associated strips being provided for recognition of lO-dollar bills. As explained in detail hereinafter, each of the pairs of wipers 107, 113 and 114 cooperates with three of the electrical conductor strips on the drum 42, the wipers 113 cooperating with the strips 112, 115 and 116, and the wipers 114 cooperating with the strips 116, 117 and 118. Preferably, a selector switch is connected to these three pairs of wipers 107, 113 and 114 to connect a selected pair to the comparison and logic parts of the device, so that one set of comparison and logic parts may recognize a plurality of denominations. For a totally automatic machine however, or for a similar currency sorting machine, a plurality of sets of comparison and logic parts are preferably used.

As illustrated in FIG. 11, the shapes of the conductor strips on the drum are chosen from a knowledge of the predetermined detection signal which would be generated by the magnetometer when pieces of printed currency of known validity and denomination were moved past the magnetometer head by the belts 26 and 27. While the shapes of the conductor strips are selected, they are parameters of these predetermined detection signals because the predetermined detection signals are first recorded and then employed as explained hereafter as the basis for proper selection of the shapes of the conductor strips. An auxiliary group of switch wipers is connected to the conductor strips as illustrated in FIGS. 4 and 5 with the auxiliary switch wipers 119 connected to the conductor strips 110, 112, 116, and 118, and auxiliary wipers 120 being connected to the conductor strips 111, 115, and 117. The wipers 119 are connected to one of the output leads of the magnetometer. and the wipers 120 are connected to the other output lead of the magnetometer so that at any given instant, the output of the magnetometer is applied to the conductor strips on the drum with one polarity of the output being applied to the wipers 119 and the other polarity to the wipers 120. Referring for simplicity to FIG. 10, and assuming that the wiper 120 is either positive or negative with respect to the wipers 119 it will be seen that the polarity of the detection signal conducted to the pickup wipers 107 will be determined by the rotational position of the drum 42 since contact between wipers 107 and switch projections 108 will conduct one polarity of the detection signal, and contact between the wipers 107 and the switch projections 109 will conduct the detection signal to the wipers 107 with opposite polarity.

It should be here mentioned that recognition of a piece of printed matter is accomplished with the apparatus of FIG. by integrating the detection signal from the magnetometer over the period that a piece of currency passes the detection head. Since this magnetometer is, however,.sensitive to the rate of change of the density of the pattern of printed indicia on the currency, the unaltered integral of the detection signal during the period a bill is examined would give a zero integration sum. The function then of the counterpart switching elements on the drum 42 is to select known portions of the signal a bill is expected to give, reversing the polarities of these portions where desired, and integrating the detection signal after adjusting the polarity of its parts. In other words, when the wipers 119 are positive with respect to the wiper 120, and a valid l-dollar bill is being examined, the wipers 107 will contact switch projections 108 yet when the polarity of the detection signal reverses so that wiper is positive with respect to wiper 119, the wipers 107 will contact the switch projections 109.

It will thus be seen that the conductor strips 110, 111, and 112 on the drum 42 together with the switch wipers 107 form a physical counterpart of the detection signal which a valid 1- dollar bill is known to give, this physical counterpart operating to permit the production of a high integration sum by integrating the signal between wipers 107 if the polarity of the detection signal generated by the magnetometer varies in the same manner as the predetermined detection signal from which the shape of the conductor strips has been derived.

To further illustrate the manner of selection of the shape of the conductor strips on the drum 42, reference is made to FIG. 11 in which the upper four curves represent as indicated portions of the waveform of the detection signal which may be generated by a particular magnetometer when four pieces of currency of known validity and of different denominations are examined by the magnetometer. It should be noted that these four waveforms may represent the signals from individual 1, 5, l0, and 20 dollar bills, but in fact there are at least three kinds of 5 dollar bills for instance in general circulation which produce three different signals, and when a 5 dollar bill is to be recognized, all three of these signals should be recognized while rejecting the signals which would be produced by all kinds of bills of other denominations. Each waveform indicates the amplitude and polarity of each detection signal plotted against the position on the bill at which the amplitude is generated. The characters in the lower portion of the drawing show corresponding switching patterns which could be derived from the wave shapes for recognizing each indicated denomination of currency. While it is understood that threeshaped conductors are employed for regulating the polarity of the detection signal for each denomination, the simple graphs are shown as indicating the use of the three conductor strips to effect regular polarity where a projection projects upwardly from the median line and reverse polarity when the projection extends downwardly. With this in mind consider the loca tions of projections 121, 122, and 121a in the pattern for the recognition of a l-dollar bill. It will be noted that an integrator connected between projections 108 for regular polarity at area 121 will accumulate a positive integral whether the bill being examined is a l, a 10 or a 20, but that if the bill being examined is a 5 the integrator will accumulate a negative sum thus discriminating against a 5. At a later time than the integrator is connected between projections 108 for regular polarity at area 122, the integrator will again accumulate a positive sum for a l-dollar bill but will accumulate substantially no sum for the 5 and large negative sums for the 10 and 20. When the integrator is connected for reverse polarity at area 121a, the integrator will accumulate substantially nothing from a 10 or 20 dollar bill; it will accumulate a large positive sum from the negative signal of the l-dollar bill, and it will accumulate a large negative sum from the positive signal of the 5-dollar bill. Proper selection of the positioning and the direction of these projections will result in the recognition of any one denomination and the refusal to recognize bills of any other denomination.

This proper selection of the positioning and direction of the projections is accomplished as follows: The detection signals from each kind of bill of each denomination are recorded as time line graphs for instance during the period when the bills pass the head of the magnetometer; portions of this period are then selected during which the l-dollar signals, for instance, differ substantially from at least one of the remainder of the signals; the selected portions are then divided into two groups where (l) in the period portions of the first group, the l'dollar signal is substantially more positive than the other signal and (2) in the period portions of the second group, the l-dollar signal is substantially more negative than the other signal the switching projections are then reversed for the two groups, the projections represented at 121 and 122 in FIG. 11 being in one group while the projections represented at 121a in FIG. 11 being in the other group.

Thus, regular polarity projections may be selected for recognizing the l-dollar bill where: (l) the l-dollar signal is strongly positive and any one of the remainder of the signals is strongly negative, slightly negative, zero, or slightly positive, (2) the l-dollar signal is slightly positive and any one of the remainder of the signals is strongly negative, slightly negative or zero, (3) the l-dollar signal is zero and any other signal is strongly or slightly negative or (4) the l-dollar signal is slightly negative and any other signal is strongly negative. Similarly, reverse polarity projections may be selected where: 1) the 1- dollar signal is strongly negative and any one of the remainder of the signals is strongly positive, slightly positive, zero, or slightly negative, etc. The positions and polarity of the projections in the period of the signals are preferably chosen where the signal to be recognized differs substantially from all other signals, but in many instances this is not possible and positions must be chosen where it is possible to discriminate against only a few or only one of the undesired signals.

PREFERRED COMPLETE SYSTEM Referring now in detail to FIG. 10, the elements and subcombinations described above are here illustrated in combination together with various electronic means for controlling the complete system. Here the magnetic detection head 100 shown in FIG. 7 together with its backing plate 101 are shown enclosed in a double-magnetic shield 102 in the brackets 38 and 37, and a reversible drive motor 103 is shown driving the shaft 23 and the counterpart drum 42 through the friction clutch 47. The magnetic head is shown connected to a conventional head drive oscillator 104 which supplies the alternating input current to the leads 95 of the detection head. The magnetic head 100 is also connected to a conventional tuned amplifier and synchronous detector 105 by which the detection signal from terminals 96 of the head are amplified and converted to a detection signal delivered to leads 106 with the detection signal being a direct current signal the amplitude of which is proportional to the quantity of measurable flux flowing in the magnetic conductor of the head of FIG. 7 and the polarity of which at any instant is determined by the direction measurable flux is flowing in that magnetic conductor. For the sake of convenience, the complete magnetometer which produces this detection signal is referred to as comprising a flux sensing head portion thereof which is mounted adjacent to the path between the belts 26 and 27 and a power means portion thereof which includes the head drive oscillator, tuned amplifier and synchronous detector which may be located either adjacent to the head or at a substantial distance therefrom.

As indicated above, the switch wipers 107 in FIG. 10 are connected to an integrator 123 which is arrangedhere to be set initially at a high voltage permitting the detection signal to integrate it downwardly during the period a bill is examined. The integrator is initially set at a fairly high voltage and connected to the switch wipers 107, as explained hereafter, so that a signal which is to be recognized will integrate the integrator sum downwardly to a lower voltage while undesired signals will integrate the sum upwardly to a higher voltage. The integrator is preferably provided with latching means for latching the integrator sum at a high voltage at any time that the voltage increases more than a predetermined amount from the voltage initially set. Such latching means may be provided by a Zener diode connected to turn on a tube when the integrator voltage exceeds the switching voltage of the diode with the tube being connected to the integrator to raise the in tegration sum additionally, when the tube is turned on.

The voltage accumulated by the integrator, which represents the integral of the signal conducted to the integrator, is conducted to a voltage comparator 124 by lines 125. The comparator is arranged to compare the voltage across lines 125 to a preset low voltage and conduct current to leads 126 when the signal from wipers 107 has integrated the voltage in leads 125 down to the preset voltage of the comparator. This current in leads 126 from the comparator is conducted to recognition logic 131) through the coil of a normally open relay 131 to close the relay 131 responsive to current in leads 126. It should be noted that the switch terminals 68 on the stop actuated switch of FIGS. 5 and 6 are connected in parallel with the switch terminals of relay 131 so that either the relay 131 or the switch 68 when closed can maintain a normally open relay 133 in logic energized through a pair of contacts 134 of relay 133. At all times that relay 133 is energized, it maintains a pair of its contacts 135 closed permitting a relay 136 to be energized therethrough when switch terminals 76 and 77 of the cam operated switch 70 close just prior to termination of a revolution of the drum 42. This relay 136 has a pair of switch terminals 137 thereof the closing of which is the act by which the logic circuit 130 recognizes a bill which has been tested by the apparatus. Accordingly, it should be noted that at any time when the drum 42 is moving and the switch contacts 68 are thus closed, the switch contacts 68 will maintain the relay 133 energized to thereby permit subsequent actuation of the relay 136 at the end ofa cycle. When one of the stop members 61 contacts a stop screw 64 thereby opening the switch contact 68, the relay 133 will be deenergized unless the comparator 124 has energized the relay 131 indicating that the preceding area of the bill being examined had accumulated a sufficient integral to actuate the comparator.

If at any time the drum 42 is stopped and the switch 68 opened, when the integrator 123 has not yet accumulated a sufficient integral to actuate the comparator, the relay 133 will be deenergized because both the switch 68 and the relay 131 are open, and the relay 133 will remain deenergized until the drum 42 returns to its starting position and switch contacts 72 and 71 are closed; note that this is the only way relay 133 can be reenergized since its own contacts 134 are now open. As soon as relay 133 is deenergized, a relay 139 in the logic 130 will be energized through contact 1140 of the relay 131, and relay 139 has a pair of normally open contacts 141 the closing of which is the act by which the logic circuit 130 re jects a test bill.

It should be noted that the provision of the latching means with the integrator adds substantial flexibility to the abovementioned method of selecting the positions and directions of the switch projections on the drum 42. Thus any particular denomination bill may be rejected by positioning the projections so that the first part of its signal which is conducted to the integrator causes the integrator to latch at high voltage.

It should be recalled that a pair of switch contacts 74 and 75 on the cam-actuated switch 70 are closed at desired portions of the period of rotation of the drum 42, and it should be noted that these switch contacts close a circuit to the tuned amplifier 105 whereby the gain of the amplifier is increased until the contacts are again opened. This permits selective emphasis of local portions of the detection signal produced by the magnetometer.

Indicated centrally of FIG. 10 is a mulltiterminal relay 142 having an energizing coil 143 and a plurality of arms 144-148. In the relay 142 the relay arms move to their arms upward position when the coil 143 thereof is not energized. It will be noted that the coil 143 of relay 142 is connected to be energized through the normally open contacts 69 of the stop-actuated switch shown inFIGS. 5 and 6 with the result that the relay coil 143 is energized and moves the arms 144-148 to their arms downward position every time that the drum 42 comes to rest. When the drum is moving and the arms are in their upward position, the arms 144 and 145 conduct the detection signal from the magnetometer to the switch wipers 119 and 120 on the counterpart drum 42; at the same time the switch arm 146 is inactive and the arms 147 and 148 conduct the polarity modified signal from switch wipers 107 on the drum 42 to the integrator 123.

When the drum comes to rest and the switch contacts 69 are closed, the arms 147, and 148 are connected to a voltage divider to reset the initial level of the integrator so that it can start to integrate downwardly when the drum begins to move again; at the same time switch arm 146 is connected to a source of positive voltage in connection with a timing capacitor 149 to prevent the resetting of the integrator from deenergizing the relay 133. Thus, high voltage is conducted to the comparator 124 with a time delay determined by the capacitor 149 to insure that the comparator energizes the relay 131 after the time delay. During the time delay the relay 131 may be energized or deenergized depending upon the prior condition of the comparator and integrator, and the period of the time delay is selected to be just shorter than the time necessary for the resetting of the integrator to influence the comparator. Also when the coil 143 is energized the arms 144 and 145 thereof move to their downward position to conduct the next portion of the detection signal from the magnetometer to a thyroton 150 which is in turn connected to the stop release relay 66. Generation of the next portion of the detection signal from the magnetometer will thereby actuate the stop release relay 66 and restart motion of the counterpart drum 42 in synchronization with motion of a piece of currency along its path under the magnetometer head.

It should also be noted in FIG. that an additional actuating circuit is provided for the thyroton 150, this circuit being connected through switch contacts 72 and 73 and having a fairly long time constant, the purpose of the circuit being merely to actuate the thyroton and return the counterpart drum to its starting position in case of bill failure to pass, when the drum 42 stops for an unduly long time at some intermediate point in the cycle.

Finally it should be noted in FIG. 10 that a triode 151 has its plate and cathode connected to supply negative feedback to the direction terminals 96 of the magnetometer head. The grid of triode 151 is connected to the output of the power means portion of the magnetometer through a switch 152 to increase the amount of negative feedback to the head portion responsive to increases in the magnitude of the direct current detection signal, whereby when the switch 152 is closed the triode I51 automatically attenuates to a large degree the production of any detection signal by the magnetometer. The switch 152 connecting the grid of the triode 151 to the output of the magnetometer is a normally closed switch which is opened responsive to closing of the start 31 adjacent to the input tray of the device (see FIG. 1), and triode 151 is selected to have a very long leakage time for its grid potential so that the triode 151 will maintain a substantially constant level of negative feedback for a substantial period of time after operation of the apparatus is started responsive to closing of the switch 31 and the opening of switch 152. In order to increase the leakage time of the grid potential of triode 151, a capacitor 151a is connected to the grid between the grid and the switch 152. It should also be noted that means are provided for holding the switch 31 closed and the switch 152 open until the closing of switch contacts 71 and 72; this insures that the switch 31 will not stop operation of the drive motor before completion of a cycle and it assures that the switch 152 will not provide automatic suppression of the detection signal when a detection signal is desired. It should be noted further that a substantial advantage is provided through the use of this means for automatically attenuating the detection signal produced by the magnetometer particularly since the magnetometer employed in this invention and forming a part thereof is so highly sensitive. Small changes in the magnetic field around the device could produce an excessively biased detection head through which magnetic flux flowed continually thereby producing a detection signal which was not the desired parameter of the pattern of printed indicia on pieces of currency being examined by the device.

We claim:

1. A flux sensing head portion for a magnetometer which comprises a magnetic shield having a compartment therein and an aperture therethrough communicating with said compartment, a backing plate constructed of a low reluctance material mounted adjacent to said aperture outside of said compartment and positioned to permit passage of material to be examined between said aperture and said backing plate, a magnetic conductor mounted inside of said compartment in said shield and having a flux receiving end positioned adjacent to said aperture to receive magnetic flux extending through said aperture from said backing plate, said conductor also having a flux returning end positioned to return to said backing plate flux flowing through said conductor, and magnetic flux measuring means mounted adjacent to said magnetic conductor between the ends thereof for measuring magnetic flux passing through said conductor.

2. The apparatus of claim l in which said flux returning end of said magnetic conductor is connected to said magnetic shield.

3. The apparatus of claim I in which said magnetic conductor is generally U-shaped having a base portion remote from said aperture and a pair of leg portions extending therefrom toward said aperture and terminating in said ends thereof adjacent to said aperture, and a pair of said measuring means are provided, one mounted adjacent to each of said leg portions for measuring the magnetic flux flowing in said leg portions, said pair of measuring means being connected together to measure the sum of the magnetic flux flowing in both of said leg portions.

4. The apparatus of claim 1 in which said magnetic shield has portions thereof adjacent to said aperture which are generally parallel to said backing plate and terminate in edge of said aperture which are sloped away from said magnetic conductor as they extend into said compartment.

5. The apparatus of claim 1 in which said magnetic conductor contains an intermediate portion thereof made of a pair of separate magnetic conductor strips which are generally parallel and connected together at their adjacent ends; said magnetic flux measuring means comprises a pair of coils, one encircling each of said strips, oppositely polar drive leads connected to said coils to establish a closed magnet circuit through said strips and said connected ends thereof responsive to electric current in said drive leads, and oppositely polar detection leads connected to said coils to receive current from said coils responsive to the establishment of similarly directed streams of magnetic flux in said strips.

6. In a magnetometer having a head portion and a power means portion connected to said head portion with said head portion having a pair of output leads adapted to carry a direct current signal component responsive to magnetic flux sensed by said head portion and said power means portion having input leads connected to said head output leads and output leads for conducting a direct current signal which is a parameter of said direct current signal component and normally open switch means adapted when closed to operate equipment when it is closed for a predetermined time interval responsive to said direct current signal the improvement comprising a normally closed switch connected to said normally open switch to be opened responsive to closing of said normally open switch, an electron tube having a plate, a cathode and a low current leakage grid for controlling current flow between said plate and cathode, means for connecting said plate to a source of electrical potential, means for connecting said grid to one of said output leads of said power means portion of said magnetometer through said normally closed switch to conduct said direct current signal to said grid to cause current flow through said tube, a capacitor connected to said grid in parallel with said cathode for maintaining electrical potential on each grid with respect to said cathode for a substantial period of time after said normally closed switch is opened, and means for connecting said cathode to one of said output leads of said head portion of said magnetometer for conducting current from said cathode to said head in opposition to said direct current signal component. 

1. A flux sensing head portion for a magnetometer which comprises a magnetic shield having a compartment therein and an aperture therethrough communicating with said compartment, a backing plate constructed of a low reluctance material mounted adjacent to said aperture outside of said compartment and positioned to permit passage of material to be examined between said aperture and said backing plate, a magnetic conductor mounted inside of said compartment in said shield and having a flux receiving end positioned adjacent to said aperture to receive magnetic flux extending through said aperture from said backing plate, said conductor also having a flux returning end positioned to return to said backing plate flux flowing through said conductor, and magnetic flux measuring means mounted adjacent to said magnetic conductor between the ends thereof for measuring magnetic flux passing through said conductor.
 2. The apparatus of claim 1 in which said flux returning end of said magnetic conductor is connected to said magnetic shield.
 3. The apparatus of claim 1 in which said magnetic conductor is generally U-shaped having a base portion remote from said aperture and a pair of leg portions extending therefrom toward said aperture and terminating in said ends thereof adjacent to said aperture, and a pair of said measuring means are provided, one mounted adjacent to each of said leg portions for measuring the magnetic flux flowing in said leg portions, said pair of measuring means being connected together to measure the sum of the magnetic flux flowing in both of said leg portions.
 4. The apparatus of claim 1 in which said magnetic shield has portions thereof adjacent to said aperture which are generally parallel to said backing plate and terminate in edge of said aperture which are sloped away from said magnetic conductor as they extend into said compartment.
 5. The apparatus of claim 1 in which said magnetic conductor contains an intermediate portion thereof made of a pair of separate magnetic conductor strips which are generally parallel and connected together at their adjacent ends; said magnetic flux measuring means comprises a pair of coils, one encircling each of said strips, oppositely polar drive leads connected to said coils to establish a closed magnet circuit through said strips and said connected ends thereof responsive to electric current in said drive leads, and oppositely polar detection leads connected to said coils to receive current from said coils responsive to the establishment of similarly directed streams of magnetic flux in said strips.
 6. In a magnetometer having a head portion and a power means portion connected to said head portion with said head portion having a pair of output leads adapted to carry a direct current signal component responsive to magnetic flux sensed by said head portion and said power means portion having input leads connected to said head output leads and output leads for conducting a direct current signal which is a parameter of said direct current signal component and normally open switch means adapted when closed to operate equipment when it is closed for a predetermined time interval responsive to said direct current signal the improvement comprising a normally closed switch connected to said normally open switch to be opened responsive to closing of said normally open switch, an electron tube having a plate, a cathode and a low current leakage grid for controlling current flow between said plate and cathode, means for connectIng said plate to a source of electrical potential, means for connecting said grid to one of said output leads of said power means portion of said magnetometer through said normally closed switch to conduct said direct current signal to said grid to cause current flow through said tube, a capacitor connected to said grid in parallel with said cathode for maintaining electrical potential on each grid with respect to said cathode for a substantial period of time after said normally closed switch is opened, and means for connecting said cathode to one of said output leads of said head portion of said magnetometer for conducting current from said cathode to said head in opposition to said direct current signal component. 